The present invention relates to a solvent-free hotmelt adhesive based on a polyester polyurethane. The polyurethane is obtainable by reacting organic diisocyanates with preferably difunctional polyester polyols containing alcoholic hydroxyl groups and low molecular weight diols as chain-extending agents at an NCO:OH equivalent ratio of from 0.9:1 to 1.05:1.
Solvent-containing, 2-component adhesive systems based on polyurethanes, are used in the production of composite structures, such as laminated parts for the interior finishing of motor vehicles. Unfortunately, solvent-containing systems have many undesirable properties. They are toxic, difficult to handle, uneconomical and inflammable. Thus, they incur additional costs through the need for application units, extraction systems, solvent recovery systems, emission control systems, anti-explosion equipment, fire prevention equipment, and the like. Further costs are incurred by the energy required to remove and recover the solvent, by relatively high dosages, based on solids, by measures necessary to avoid toxic working environment concentrations and by the need for relatively heavy insurance. Toxicity is attributable to the fact that the solvents used have a considerable toxic potential. However, the reactive components are far more toxic (maximum allowable concentrations or MAC value &lt;0.02 ppm). The inflammability of these adhesive systems is reflected in the fact that they are generally grouped in risk class A I, i.e., they have a flash point of &lt;21.degree. C. The effect of all these disadvantages is that the adhesive systems are difficult to handle. Limited pot life, exact dosage of the components, and exclusion of moisture are only a few additional problems which must be taken into account during processing.
To date, the disadvantages noted above had to be accepted because there was no other adhesive system that was able to satisfy the stringent requirements involved. These requirements include high cohesion; maximal thermal stability; resistance to migration in the bonding of flexible polyvinyl chloride; deep drawability; high adhesion to a variety of materials; and easy application.
U.S. Pat. No. 4,240,861 describes an adhesive which consists predominantly of a linear dihydroxypolyurethane containing from 0.01 to 1% by weight of carboxyl groups, and from 0.1 to 10 milliequivalents of ionic groups per 100 grams of polyurethane. The polyurethane is prepared by reacting (i) dihydroxypolyesters, (ii) organic diisocyanates, (iii) chain lengthening agents, (iv) compounds containing carboxyl groups and groups having a higher isocyanate-reactivity than carboxyl groups, and (v) compounds containing ionic groups and isocyanate-reactive groups. The compositions described therein are soluble in methyl ethyl ketone and because of the ratio of components used have relatively low urethane group contents.
In view of the above-noted disadvantages of existing adhesive systems and in view of the stringent demands stated above, the object of the present invention was to provide a solvent-free adhesive which satisfied the following requirements: activatability below the damage limit of the substrates to be bonded (generally about 130.degree. C.); high thermal stability under tensile stressing (24 hours at 80, 90 or 120.degree. C., depending on the application); resistance to plasticizer migration; high adhesion level; deep drawability; and satisfactory adhesion to metal surfaces.